As is described in the U.S. patent application Ser. No. 09/390,106 entitled Method and Apparatus for a Start-Up Procedure for Digital and Analog Modems Utilizing Pulse Code Modulation for Data Transmission, filed Sep. 3, 1999, assigned to the assignee hereof and incorporated herein by reference, a startup procedure is described which details a startup sequence between digital and analog PCM modems. For both the analog and the digital modems, the receiver structures are capable of measuring channel impairments given that there is a reference training sequence transmitted by each transmitter. At the end of the startup sequence each receiver has enough information to know exactly what the channel impairments are in either direction.
Given that each receiver knows the impairments, they can subsequently design proper equalization or pre-equalization structures based on the application or the direction of the channel. For the digital modem in the upstream direction, the digital modem designs a set of parameters for the analog modem transmitter. These sets of parameters include the transmit constellation, mapping, and pre-equalizer parameters. The pre-equalizer structure itself is combined from pre-filter and pre-coder filter coefficients. All of these parameters are calculated based on the measurements by the digital modem receiver during the startup sequence and then passed through to the analog modem to be used during the data mode. The initial startup sequence enables the digital modem to properly measure the upstream channel and then design the parameters that are then transmitted to the analog modem and used in data mode.
As described in U.S. patent application Ser. No. 09/390,106 entitled Method and Apparatus for a Start-Up Procedure for Digital and Analog Modems Utilizing Pulse Code Modulation for Data Transmission, procedures are defined in which the digital modem updates all these parameters via a rate re-negotiation scheme in which parameter updates are transmitted to the analog modem only after exiting the data mode and reentering the startup mode. This requires switchover to the startup structure, resulting in an extra time penalty and a lower transfer rate in both directions. Also, different transmitter (analog modem) and receiver (digital modem) structures must be used in the upstream direction since Pulse Amplitude Modulation is utilized during the startup and PCM modulation is utilized during the data mode.
The distinction between the startup mode and the data mode is that during the startup mode data is transmitted in the upstream direction through the utilization of pulse amplitude modulation or PAM. In contradistinction, in the data mode data is transmitted in both directions via PCM modulation. In general transfer rates in data mode are much higher than transfer rates in the startup mode.
To summarize, when, after startup, transmitter parameters needed to be modified, the startup mode is again invoked which involves the use of the pulse amplitude modulation. It will be appreciated, that once having entered into the data mode, switching from the data mode to the startup mode is a time consuming because of the complete structure change for the analog modem transmitter and the digital modem receiver between PCM modulation and PAM.
In a typical situation, switching from the data mode to the startup mode can take as long as two to seven seconds, an inordinately long time to simply readjust the transmit parameters in the modems. This significantly underutilizes the bandwidth during the switchover time, thus lowering throughput.